The present invention relates to an evaporative emission control system in a fuel supply system of an automotive vehicle. More particularly, the invention relates to a system for detecting condition of a fuel evaporative emission which detects amount of the fuel evaporative emission generated in a fuel tank.
In general, evaporative emission control systems for presenting the fuel evaporative emission generated in the fuel tank from being discharged into atmosphere have not been in the automotive technologies. Such system generally absorbs the fuel evaporative emission generated in the fuel tank with an absorbent disposed within a canister, and subsequently supplies the absorbed fuel evaporative emission into an air induction system with a fresh air introduced through a fresh air inlet opening formed through the canister, by vacuum pressure in the air induction system, depending upon the driving condition of an engine.
In this type of evaporative emission control system, a pressure sensor is provided for detecting the internal pressure in the fuel tank and whereby for detecting amount of the fuel evaporative emission generated in the fuel tank (for example, in Japanese Unexamined Patent Publication (Kokai) No. 2-136558). In the conventional arrangement, the pressure sensor simply detects the pressure within the fuel tank to make judgement that the greater internal pressure of the fuel tank reflects greater amount of the fuel evaporative emission generated therein.
However, the internal space of the fuel tank is not completely sealed in gas tight fashion. Namely, the interior space of the fuel tank can be communicated with atmosphere through the fresh air inlet opening of the canister, or, in the alternative, temporarily opens to the atmosphere through a control valve disposed within the canister.
Therefore, the pressure within the fuel tank can be significantly influenced by the atmospheric pressure. Namely, irrespective of generation of the fuel evaporative emission, the pressure within the fuel tank can be fluctuated by the atmospheric pressure.
Accordingly, in the above-mentioned method (the method to simply detect the internal pressure within the fuel tank and to derive the fuel evaporative emission generation amount on the basis of the internal pressure in the fuel tank), erroneous detection can be caused to make judgement that a large amount of the fuel evaporative emission is generated despite of the fact that a small amount of fuel evaporative emission is indeed generated, when the pressure is risen by the influence of the atmospheric pressure as set forth above.
Conversely, if the pressure in the fuel tank is lowered by the influence of the atmospheric pressure, erroneous detection of that small amount of fuel evaporative emission is generated despite of the fact that large amount of fuel evaporative emission is indeed generated, can be caused. Reference may be made to copending U.S. Patent application entitled "Self-diagnosis system in evaporated fuel gas distribution preventing system" filed on basis of Japanese patent application No. 3-75413 (of the filing date Apr. 8, 1991); and copending U.S. patent application entitled "Gaseous fuel flow rate detecting system" filed on basis of Japanese patent application No. 3-75414 (of the filing date Apr. 8, 1991), respectively filed in behalf of Nippon Denso Co., Ltd. (the assignee of the present application).